1. Field of the Invention
The invention relates to the field of power electronics. It proceeds from a method and a device for correcting the DC offset of a converter in accordance with the preamble of the first claim.
2. Discussion of Background
In U converters, that is to say converters having a DC intermediate circuit, which feed a power transformer on the AC voltage side, problems arise with a DC voltage component (DC offset) attached to the AC voltage. Although, by comparison with the amplitude of the AC voltage (for example 7.5 kV) this DC offset is very small (for example a few volts or fractions thereof), this DC offset can bring the transformer to saturation. This substantially reduces the power to be transmitted, or can even render operation impossible. The following solutions have been proposed to date to reduce these influences:
On the one hand, structural measures are undertaken on the transformer. In particular, an air gap is provided, or induction measurement probes (for example Hall probes) are inserted into the transformer and the DC offset is corrected as well as possible. However, these measures are comparatively complicated and expensive and are capable of controlling the DC offset only to a certain minimum. The structural interventions on the transformer are attended by relatively large losses and noise emissions. These disadvantages are no longer acceptable, in particular for high-power transformers.
On the other hand, attempts have been made to measure and correct the DC offset by means of high-precision DC current transformers, which are therefore also expensive. However, this measure likewise produced only unsatisfactory correction, and the control was correspondingly complicated and expensive.